Sintering may be defined as the thermal treatment of a powder or compact for the purpose of bonding the particles together to create a solid article.
In certain applications where the powder is comprised of a mixture of powders of at least two distinct materials with different melting points, the powder mixture is compacted into a porous (“green”) body. This body is heated above the melting point of the lowest melting constituent and a portion of the compacted loose powder mixture is liquified. After maintaining the body at the sintering temperature for a predetermined time, the material is allowed to cool and the liquid solidifies and “cements” the body into a densified useful structure. Examples of such systems are copper/tin, iron/copper, and tungsten carbide/cobalt.
In such processes, the densification of the compacted body takes place in the presence of a liquid phase, and such sintering processes are termed “liquid phase sintering” (LPS). In some systems, particularly the consolidation of “hard metals” such as tungsten carbide and other ceramic particles, LPS is sometimes called conventional sintering. In LPS processes it is beneficial to have a certain minimum amount of liquid phase present at sintering temperature to assure transport of the binder phase to accomplish uniform distribution and densification. It is also generally beneficial to restrict the amount of liquid phase present in order to avoid part shape deformation and grain growth.
This liquefaction enables, for instance, increased mass transport, particle rearrangement, development of a skeletal structure, and densification. It is generally thought that this is accomplished by rounding of the particles as the external irregularities are liquefied, and by the migration of this liquid to fill the voids. Upon cooling, recrystallization and often grain growth occur. Porosity, as a percentage of the whole volume, may decrease due to densification of the structure. The rate of densification may be influenced by, for example, sintering temperature, sintering time, sintering pressure, sintering atmosphere, and weight fraction of the binder constituent present.
Liquid phase sintering of conventional hardmetals such as tungsten carbide—cobalt (WC—Co) compacts is generally performed at sintering temperatures that range from 1325° C. to 1475° C.
As the WC—Co compact is heated during sintering of WC—Co hardmetals, the cobalt will start to behave like a very viscous liquid at about 700° C. and diffusion will increase with increasing temperature as Co viscosity correspondingly decreases. The grease-like behavior and viscosity of Co metal is believed to create capillary attractor forces resulting from the strong propensity of Co to wet as much WC surface as possible. This results in a rearrangement of WC particles and the composite begins to shrink even before the first liquid phase has formed.
At 1275° C., the Co binder metal begins to dissolve the WC particles and a ternary eutectic reaction begins to form a Co—W—C alloy. As temperature continues to increase, the increased surface wetting, liquefication, and capillary forces cause continued particle rearrangement and shrinkage of the powder mass into the shape of desired articles as grain boundaries move through the interface between the WC grains and Co binder phase.
High density, uniformity, and WC stoichiometry in the sintered part are basic requirements for WC—Co microstructural integrity and strength. Ensuring proper local carbon balance during liquid phase sintering, which eliminates the formation of the brittle carbon-deficient CO3W3C eta phase and carbon porosity caused by too much carbon is also important in providing the fracture toughness of WC—Co materials. Eliminating strength-robbing porosity and grain growth in the microstructure can be accomplished through selection of an appropriate sintering temperature and pressure. For example, the temperature must be high enough to liquefy an adequate amount of material to accomplish the mass transfer necessary to fill the pores between particles while maintaining the temperature low enough to avoid WC overdissolution that causes grain growth. To the extent that capillary forces are insufficient to provide densification to close to theoretical density, external pressure may be applied.
In conventional sintering, typically small percentages (3-18 wt %) of cobalt are mixed with WC. The cobalt binder plays a role in densification and its uniform distribution is desired in order to achieve uniformity in WC—Co microstructures. Microstructural defects are commonly found in sintered WC—Co parts. A general cause is inherently imperfect blending (even for long periods of time) of WC and Co powders that are of approximately equal diameters. It is desired that this process will encapsulate (or at least associate) each WC particle with just the right amount of Co such that the Co-to-WC ratio is essentially uniform throughout the mix. Statistically, it is highly unlikely that this result can be achieved because cobalt is not available in small enough nanoparticles to blend uniformly with the WC particles. Cobalt oxygenation, explosive pyrophoric reactions, and particle agglomeration are among the barriers to their availability.
The consequence is a WC—Co mixture with Co-rich and Co-poor areas. The liquid phase occurs first in the Co-rich zones, and the cobalt, unsaturated with WC, seeks thermodynamic equilibrium by (a) consuming nearby smaller WC crystals (the smallest ones may be totally consumed) and (b) by mobilizing unsaturated Co over long distances toward Co-poor zones to dissolve more and more WC until saturation is reached. Thus, a higher temperature than that necessary to create the liquid phase is needed to liquefy and transport the cobalt to Co-poor zones where it is required for equilibrium and for sufficient Co liquid to wet the WC particles.
Combating the effects of this uneven Co distribution is typically done using (a) very long ball-milling times, (b) higher sintering temperatures, and (c) longer sintering times. The ball milling tends to reduce many of the WC particles into fines, which are preferentially dissolved by Co during heating. The latter two measures do help spread the binder phase and normalize the distribution of the liquid Cobalt during sintering, but they also increase the dissolution of WC. In addition, some of the Co will penetrate the WC particles along their grain boundaries because of the WC/WC interface energy is higher (more positive) than the interface energy of WC/Co, at least as long as grain boundaries are present with interface angles nearly perpendicular to the surface. Upon cooling, the saturated WC—Co solution precipitates WC, preferentially nucleating and recrystallizing WC onto the adjacent remaining larger undissolved WC crystals, creating the undesirable Ostwald ripening (grain growth) phenomenon as solidification takes place. This grain growth proceeds until the temperature is decreased to below the 1275° C. ternary eutectic of the Co—W—C system. FIG. 1 shows the pseudobinary WC—Co phase diagram. Sintered densities of nearly 100% are commonplace for WC—Co materials.
Increasing sintering temperatures thus aids binder mobility but also causes excessive WC dissolution, resulting in unwanted grain growth. There is a tradeoff between sintering temperature and sintering time that must be carefully balanced. The maximum temperature must be high enough to liquefy enough material to accomplish the mass transfer necessary to fill the pores between particles (compromises structural strength) while trying to avoid too high a temperature for too long a time to avoid grain growth (which also reduces structural strength).
Since control of sintering temperature is one major aspect for high quality hardmetal microstructures, alternative sintering techniques have been employed. These techniques include the investigation of shortened sintering times (e.g. microwave sintering) and use of gas pressures (e.g. hot pressing, hot isostatic pressing [HiP], and the Ceracon and Roc-Tec sinter-forging methods) to achieve consolidation at lower temperatures.
Another approach used in consolidating conventional hardmetals is to increase the weight fraction of the binder such as cobalt. This can be in the range of 18-25 wt %. This not only increases the amount of liquid present but can have the beneficial effect of increasing the toughness of the structure. However, this approach has two significant drawbacks and is therefore generally avoided. First, increasing the weight percent of binder diminishes the weight percent of WC (the wear-resistant phase) in the structure and diminishes wear resistance accordingly. Second, increasing the amount of binder also dissolves more WC, contributing significantly to grain growth during cooling.
Further, the only means to improve the wear resistance of conventional carbides (while retaining the high fracture toughness of the WC—Co substrate) for the past seventy years has been to (a) continuously refine and improve conventional powder and consolidation processing methods, (b) to add thin wear-resistant coatings, and (c) to laminate harder materials onto a WC—Co substrate. Improving conventional WC—Co microstructures is a delicate balance of time, temperature, grainsize, and other product and process parameters. Incremental improvements in conventional carbides have been achieved over the past fifty years through better sintering temperature control and the use of higher purity, highly uniform WC and Co starting powders. Since the introduction of external coatings over thirty years ago, improvements in wear resistance of materials with the toughness of WC—Co has been slowed almost to a halt.
While these techniques have reduced the problems that occur in liquid phase sintering of conventional hardmetals, there nevertheless remains an unmet need for a method of producing particles with properties that allow for uniform properties throughout the WC and binder powders upon sintering and articles formed from such particles.
To avoid the previously described drawbacks, the invention provides a method of consolidating by liquid phase sintering a new class of designed-microstructure particulate materials with unprecedented combinations of property extremes called Tough-Coated Hard Powders (TCHPs, or EternAloy®). This novel family of sintered particulate materials is comprised of one or more types of superhard Geldart Class C or larger ceramic or refractory alloy core particles having extreme wear resistance, lubricity, and other properties which are (1) individually coated with nanolayers of a metal compound having a relatively higher fracture toughness, such as WC or TaC, and (2) coated again with a second layer comprising a binder metal, such as Co or Ni. The combination of multiproperty alloys within the TCHP sintered structure allows the combination of normally conflicting performance extremes, including, but not limited to toughness, abrasiveness, chemical wear resistance, and light weight, at levels heretofore to provide materials with superior properties unavailable from the sintered homogeneous powders. TCHP materials are disclosed in U.S. Pat. No. 6,372,346 to Toth, which is incorporated by reference herein.
The process of the present invention allows the integration of thermodynamically incompatible material phases and property extremes in a single material. Thus, TCHP materials can be engineered to combine hardness approaching that of diamond with fracture toughness greater than that of tungsten carbide, and weight approximately that of titanium. As a result, TCHPs can significantly exceed the wear resistance of conventional metal cutting and forming tools; abrasives; friction and wear products and thermal coatings; and automotive, aerospace, heavy industrial, and defense components.